Identification of the site of interaction of the dihydropyridine channel blockers nitrendipine and azidopine with the calcium-channel alpha 1 subunit.

The dihydropyridine binding site of the rabbit skeletal muscle calcium channel alpha 1 subunit was identified using tritiated azidopine and nitrendipine as ligands. The purified receptor complex was incubated either with azidopine or nitrenidpine at an alpha 1 subunit to ligand ratio of 1:1. The samples were then irradiated by a 200 W UV lamp. The ligands were only incorporated into the alpha 1 subunit, which was isolated by size exclusion chromatography and digested either by trypsin (azidopine) or endoproteinase Asp-N (nitrendipine). Each digest contained two radioactive peptides, which were isolated and sequenced. The azidopine peptides were identical with amino acids 13-18 (minor peak) and 1428-1437 (major peak) of the primary sequence of the skeletal muscle alpha 1 subunit. The nitrendipine peptides were identical with amino acids 1390-1399 (major peak) and 1410-1420 (minor peak). The sequence from amino acids 1390 to 1437 is identical in the alpha 1 subunits of skeletal, cardiac and smooth muscle and follows directly repeat IVS6. These results indicate that dihydropyridines bind to an area that is located at the putative cytosolic domain of the calcium channel.